Properties of Magmas

In this chapter the properties of magma are reviewed in order to understand how magmas behave as they rise to the Earth’s surface, why magmas erupt coherently as lavas or explosively as pyroclastic deposits, and how they behave subsequently. The process of magma production and transport to the Earth’s surface is an important mechanism through which the internal heat of the Earth is dissipated, ultimately to space. The dissipation of thermal energy occurs through the transport of heat, mass, and momentum to the surface. The efficiency of this heat and mass transfer depends on the characteristics of magmas, the volumes transferred, and the history of transport and cooling through the lithosphere. This leads to the wide spectrum of eruption styles observed on Earth. Igneous rocks are the products of the cooling of magma, and this can occur at any stage during the ascent of the magma. Usually, only a small proportion of the magma generated at the source will actually reach the surface. In this chapter, we briefly introduce the compositional variability of magmas and their classification, and then consider specific properties that are relevant to their rheological and eruption behaviour. Of particular importance in understanding the rheology of magmas is their viscosity, which is controlled by many variables, including pressure, temperature, chemical composition (especially volatile and silica contents), crystal content, and bubble content which can affect their fluid flow states. Finally, we provide a brief introduction to lava flows and the factors that control their flow or rheological behaviour and morphological forms, prior to their detailed discussion in Part B.